Field Analysis:Is this podzolised


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SASSA Home PageField Analysis Home PageField Interpretation ⇒ Has this deposit been affected by podzolisation?


[edit] Has this deposit been affected by podzolisation?

Podzolisation is a soil-forming process that results in the formation of a podzol soil profile. Podzolisation involves the movement of iron, aluminium, and organic matter down through the profile. This results in the formation of a grey, ashy depleted horizon (Ea) below the topsoil and beneath this a reddened or darker horizon (Bs) enriched with iron and organic matter. In archaeology podzols are important as:

  • Indicators of former environmental conditions (buried podzols)
  • They may overprint archaeological deposits obscuring evidence of depositional stratigraphy and creating soil horizons that can be mistaken for depositional stratigraphy
  • The down profile movement of organic matter can result in contamination by "younger" carbon for the pruposes of radiocarbon dating.
  • The movement of Fe, Al and other metallic elements could affect the magnitude and patterning of anthropogenic element enhancements.
Buried podzol SW Ireland
Buried podzol SW Ireland

[edit] Indicators of podzolisation

When attempting to identify the effects of podzolisation in the field a checklist of features that can be looked for/considered are:

  • A free-draining soil - podzols tend to develop in acid free-draining materials, however, iron deposition and panning can impede the drainage of water through the profile and lead to the development of mottles associated with waterlogging (gleying).
  • Acid vegetation such as heath plants and conifers.
  • The occurence of a series of identifiable soil horizons characteristic of a podzol.
Typical podzolic profile
Typical podzolic profile

A classic podzolic soil profile will consist of (Avery, 1980):

  • A distinct horizon of acid organic matter (O or Ah).
  • A grey, ashy looking Ea horizon consisting of uncoated sand grains,
  • A dark Bh horizon containing translocated organic matter.
  • A reddened Bs horizon containing enhanced levels of translocated iron.
  • One or more thin iron pans in the Bs and or underlying horizons.
  • The Bh, and if this is absent the Bs horizon, may contain tiny aggregates of black or brown organic matter between the sand grains.

However, there can be considerable variation in the sequence of horizons present. The organic, Ah or O horizon and the grey (albic) Ea horizon may be lost if the profile has been ploughed. The dark Bh horizon may also be destroyed by plouging or may not be well enough developed to be visible as a separate horizon. The reddened Bs horizon is often the most prominent feature but again may not be well developed or may be affected by gleying, whilst iron pans are absent from many types of podzol soil.

[edit] Uncertainties in identifying podzolisation

Uncertainties in identifying podzolisation can arise from:

  • Confusion with sequences of anthropogenic deposits, for example ash layers may superficially look like a depleted eA horizon and when these overlie reddened burnt materials they could be confused with a podzolic soil profile.
  • Not all podzols contain an ashy Ea horizon or have iron pans, all however should have an iron enriched Bs horizon.
  • Truncated podzols can be vey difficult to identify as the characteristic sequence of horizons may be lost.
  • Weathered soil B horizons (Bw) may also be reddened. However, Bs horizons tend to have redder hue, lack moderately or strongly developed blocky structures, have a weak consistence when moist or dry and feel greasy or smeary when very moist (Avery, 1980).
  • In some cases the iron enriched Bs horizon may also be gleyed resulting in mottling of the soil colour.

[edit] How does the SASSA Field Tool make this interpretation?

A printable recording sheet for SASSA's 'Has this deposit been affected by podzolisation?' can be found here

A high score tends to confirm that this context / deposit has been affected by podzolisation. To score highly you need:

  • A freely or excessively drained soil. This question accounts for 20% of the maximum total score.
  • A series of horizons (grey and / or reddened) that meet particular colour and morphological requirements (Avery, 1980; FAO, 1998; Soil Survey Staff, 2006). This set of questions accounts for 80% of the maximum total score.
    • For an ashy grey horizon the moist soil colour should have:
      • a value of 6, 7, or 8 and a chroma of 4 or less, or
      • a value of 5 and a chroma of 3 or less, or
      • a value of 4 and a chroma of 2 or less.
      • a value of 4 and a chroma of 3 if the parent material has a hue of 5YR or redder.
      • The deposit colour should be dominated by uncoated sand grains, it should have a sharp or abrupt lower boundary, contain less organic matter than the horizon immediately below which may contain an iron pan and will have a lower value and a higher chroma than this horizon.
      • Not included in the interpretation tool but to be considered are weak structural development and an irregular lower boundary shape.
    • For a reddened horizon the moist soil colour should be:
      • a hue of 7.5YR, 5YR, 2.5YR or 10R with a value of 5 or less and a chroma of 4 or less, or
      • a hue of 10YR with a value of 3 or less and chroma of 2 or less.
      • It should also contain pellets of organic matter between sand grains and may contain a thin iron pan, and the overlying context colour should have a higher value and a lower chroma than this horizon.
      • Not included in the interpretation tool but should also considered are the possible presence of a pale grey albic (Ae) or dark organic rich (Bh) horizon immediately overlying this one. It should have a pH (10g soil in 10g water) of 5.9 or less.

[edit] Follow-on analyses

In the field you may wish to carry out a pH test to help confirm this is a podzol. However, you should note that pH can change over time, For example if the soil is buried beneath a calcareous medium the pH of a buried podzol may be raised significantly.

Post-excavation laboratory analysis may help to confirm podzolisation include:

[edit] References

  • Avery, B.W. (1980) Soil classification for England and Wales: higher categories. Harpenden, Soil Survey of England and Wales, Technical Monograph No. 14.
  • FAO (1998) World reference base for soil resources. Rome: ISSS-AISS-IBG.
  • Soil Survey Staff (2006) Keys to soil taxonomy. Washington: USDA, Tenth edition.

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